The invention is directed to a method of forming a gel, to heat treated native potato protease inhibitor isolate, to the use of said protease inhibitor isolate, and to a foodstuff comprising said gel or protease inhibitor isolate.
The undiluted juice from potato tuber is called potato fruit juice, whereas the diluted juice is referred to as potato fruit water. Potato fruit juice may be produced by washing and rasping potatoes and separating the starch and fibres by various techniques, such as centrisieves, hydrocyclones and decanters. Fresh potato fruit juice is a complex mixture of soluble and insoluble material comprising proteins, starch, minerals, toxic glycoalkaloids and monomeric and polymeric reactive phenols.
Fresh potato fruit juice is however not very stable. Oxidation leads to conversion of phenolic compounds into quinones which rapidly combine into a dark polymer residue. During the oxidation process the potato proteins can partially cross-link, which dramatically reduces the solubility of the proteins. The complexity and instability of the potato fruit juice makes the separation and isolation of minimally denatured or modified proteins a complicated and economically demanding process.
Native potato proteins can tentatively be divided into the following three classes: (i) the patatin family, highly homologous acidic 43 kDa glycoproteins (40-50 wt. % of the potato proteins), (ii) basic 5-25 kDa protease inhibitors (30-40 wt. % of the potato proteins) and (iii) other proteins mostly high molecular weight proteins (10-20 wt. % of the potato proteins) (Pots et al., J. Sci. Food. Agric. 1999, 79, 1557-1564).
Protease inhibitors can be divided into different groups based on their molecular weight. The different groups of protease inhibitors are identified as protease inhibitor I (molecular weight of about 39 kDa), carboxypeptidase inhibitor (molecular weight of about 4 100 Da), protease inhibitors IIa and IIb (molecular weight of about 20.7 kDa), and protease inhibitor A5 (molecular weight of about 26 kDa). The ratio of these different groups of protease inhibitors in the total potato protein depends on the potato variety. Protease inhibitors from potato have a broad range of potentially important applications. Protease inhibitors have for instance shown to be useful in the treatment of diabetes, for eliciting satiety in mammals, for reducing the risk of skin cancer, for inhibiting the growth of bacteria, and for preventing or treating inflammation on pruritis of skin and intestine, see for instance WO-A-99/059623.
One of the major drawbacks of the potato protein as presently used is that the recovery of potato protein in pure form has shown to be very difficult. Most methods of the prior art yield potato proteins in low purity, are not selective and/or are unable to separate the different functionalities.
The non pre-published European patent application 06077000.5 describes a selective and efficient process for the isolation of native potato protein and the different native potato protein fractions with a high degree of purity. This patent application also mentions that native potato protein patatin isolate and native potato protease inhibitor isolate can be used as a gelling agent in a food product.
For gel formation, various types of protein gelling mechanisms and compounds are known and applied in the prior art. Typical examples include thermogelling (for instance gelatine can form a gel after cooling a solution), acid induced gelling by denaturation and flocculation, and gel formation by partial degradation of proteins using enzymes. Conversion processes have also been described for instance by Creusot (PhD thesis, “Enzyme-induced aggregation of whey proteins with Bacillus licheniformus protease”, Wageningen University, The Netherlands, 2006), wherein a gel is formed via one step (native gel→gel) or two step (native protein→pre-aggregate→gel) gel formation processes.
Particularly in sterilised foods, a control of the gel formation is important. The currently available proteins show a wide variety of gelling behaviour, but do not show a heat independent gel formation. This would be desirable for applications in high protein foods and acid foods.
In addition, it is desirable to replace animal proteins and/or allergenic proteins such as gelatine, egg and whey/milk proteins, in consumer products.
Furthermore, it is desirable to have an easy gel formation process with mild processing.